Fix commonmark Markdown warnings in docs, part 1

Fixes warnings from rust-lang/rust#44229 when `--enable-commonmark` is
passed to rustdoc.

This is mostly a global find-and-replace for bare URIs on lines by
themselves in doc comments.

components/servo_arc/lib.rs

@@ -9,6 +9,7 @@
 // except according to those terms.
 
 //! Fork of Arc for Servo. This has the following advantages over std::Arc:
+//!
 //! * We don't waste storage on the weak reference count.
 //! * We don't do extra RMU operations to handle the possibility of weak references.
 //! * We can experiment with arena allocation (todo).
@@ -16,7 +17,7 @@
 //! * We have support for dynamically-sized types (see from_header_and_iter).
 //! * We have support for thin arcs to unsized types (see ThinArc).
 //!
-//! [1] https://bugzilla.mozilla.org/show_bug.cgi?id=1360883
+//! [1]: https://bugzilla.mozilla.org/show_bug.cgi?id=1360883
 
 // The semantics of Arc are alread documented in the Rust docs, so we don't
 // duplicate those here.
@@ -83,7 +84,7 @@ const MAX_REFCOUNT: usize = (isize::MAX) as usize;
 /// be thin or fat (which depends on whether or not T is sized). Given
 /// that this is all a temporary hack, this restriction is fine for now.
 ///
-/// [1] https://github.com/rust-lang/rust/issues/27730
+/// [1]: https://github.com/rust-lang/rust/issues/27730
 pub struct NonZeroPtrMut<T: ?Sized + 'static>(&'static mut T);
 impl<T: ?Sized> NonZeroPtrMut<T> {
     pub fn new(ptr: *mut T) -> Self {